To study effects of excavation methods on the stability of existing tunnel, a 3D numerical model of tunnel excavation is established based on the finite element software FLAC 3D taking Shenzhen Metro Line 9 as an example. The effects of excavation methods on the strain of the new and existing tunnels. The simulated results are compared with the field monitoring data. The results show that the strain of tunnel by different excavation methods obtained from simulation and field monitoring holds a similar pattern. The impacting extent of excavation method on the tunnel stability (from large to small) is full section method, step method, CD method and bilateral wall tunnel method. Comprehensively considering the cost of construction and time, the optimal excavation method for the project should be step method.